Transformer has been widely introduced into a variety of electric appliances as a basic circuit component. Referring to FIG. 1, which shows a typical representation of a conventional transformer structure. As shown in FIG. 1, a conventional transformer 1 includes a magnetic core assembly 11, a bobbin 12, a primary winding coil 13, and a secondary winding coil 14. The primary winding coil 13 and the secondary winding coil 14 are wound up and down in the winding area within the bobbin 12, and are separately isolated by a tape 15. The magnetic core assembly 11 is generally shaped in the form of an EE-core, an EI-core, or an ER-core, wherein a central leg 111 thereof is mounted in the hollow 121 of the bobbin 12 so that the magnetic core assembly 11 can achieve a magnetic coupling effect with the primary winding coil 13 and the secondary winding coil 14, and thereby perform voltage transformation.
The regulation on the leakage inductance of a transformer is an important factor for a power converter. This is because the leakage inductance of a transformer would influence the power conversion efficiency of a power converter. In order to improve the power conversion efficiency of a power converter, enormous efforts has been paid to strive for the increase of the coupling coefficient of the transformer windings, the reduction of leakage inductance, and the abatement of the power loss during voltage transformation process. In the transformer structure of FIG. 1, the primary winding coil 13 and the secondary winding coil 14 are wound up and down in the winding area within the bobbin 12, and thus the magnetic leakage induced between the primary winding coil 13 and the secondary winding coil 14 is lower, while the coupling coefficient is increased and the power loss during the voltage transformation process is abated. In this way, the power conversion efficiency of a power converter is improved.
However, the power supply system adapted for the new generation electronic product, such as LCD TV, incorporates a transformer with a leakage inductance as a mainstream circuit design rule. The current circulating in the power supply system generally flows through a LC resonant circuit comprising a leakage inductor L and a capacitive element C. In the meantime, a current having a quasi-semisinusoidal waveform flows through a field-effect transistor (FET) switch. When the current value is zero, the FET switch is turned on. After a period of a semi-sinusoidal waveform is elapsed and the current value returns to zero, the FET switch is turned off. Using such soft switch design with a resonant circuit, the switching loss and the noise of the switching element can be degenerated.
There are various ways to increase the leakage inductance of a transformer. The most common way to increase the leakage inductance of a transformer is to separate the primary winding coil and the secondary winding coil by a specific distance in order to decrease the coupling coefficient, and thereby increase the leakage inductance of a transformer. Referring to FIG. 2, a conventional transformer with a leakage inductance is shown. As shown in FIG. 2, the transformer 2 includes a bobbin 21, a primary winding coil 22, a secondary winding coil 23, and a tape 24, in which the bobbin 21 includes a first lateral plate 211, a second lateral plate 212, and a winding window portion 213. The tape 24 is wound up substantially in the middle of the winding window portion 213 and has a width d, so that the winding window portion 213 is divided into a first winding area 2131 and a second winding area 2132. The primary winding coil 22 and the secondary winding coil 23 are wound around the first winding area 2131 and the second winding area 2132, respectively, and are respectively separated from the first lateral plate 211 and the second lateral plate 212 by a first lateral tape 25 and a second lateral tape 26. By way of the isolation effect provided by the tape 24, a constant distance of electrical security can be maintained between the primary winding coil 22 and the secondary winding coil 23. In addition, a safe distance of electrical security can be maintained between the primary winding coil 22, the secondary winding coil 23 and the external conductor through the use of the lateral tapes 25 and 26. Furthermore, the larger the width of the tape 24 is, the lower the coupling coefficient of the transformer will be, and the larger the leakage inductance of the transformer will be. This would favor the regulation on the resonant circuit of the power supply system.
Although the transformer structure shown in FIG. 2 can increase the leakage inductance of transformer, there still exists numerous problems needing to be conquered. For example, the magnitude of the leakage inductance depends on the width of the tape 24 which is wound around the primary winding coil 22 and the secondary winding coil 23. Because the tape 24 is a soft material and is not possible to be thoroughly affixed, the transformer windings would bear a weak structure and would be susceptible to displacement after a long usage time. This would lead to a lower or unstable leakage inductance, and would further affect the regulation on the resonant circuit of the power supply system. Moreover, the arrangement of using the tape 24, the first lateral tape 25 and the second lateral tape 26 as an isolator requires onerous labor work to wind the tapes. Since the tape has viscosity and has a relatively small width, the laboring efforts expended on tape winding will be time-consuming and complicated. This would limit the promotion of yield and cause wastes on manpower and cost. Also, the quality of transformer would be seriously deteriorated due to the improper winding of the tapes.
More disadvantageously, because it is necessary to wind the tape 24, the first lateral tape 25 and the second lateral tape 26 around the winding window portion 213 of the bobbin 21, the area and capacity in which the primary winding coil 22 and the secondary winding coil 23 can occupy within the winding window portion 213 are significantly reduced, and the heat-dissipating performance of the transformer is worsened accordingly. Further, when the winding process and the taping process are finished, an additional tape is needed to be further coated on the primary winding coil 22 and the secondary winding coil 23 for the purpose of isolation. This would aggravate the poor heat-dissipating efficiency of the primary winding coil 22 and the secondary winding coil 23. What is worse, as the melting point of the tape is relatively low, the maximum operating temperature of transformer is unavoidably limited, resulting in a narrow applicability for the transformer.
Accordingly, there is an urgent need to develop a transformer with a leakage inductance that can overcome the deficiencies and difficulties encountered by the prior art.